1. Field of the Invention
The present invention relates generally to four wheel drive vehicles and more specifically to a four wheel drive vehicle wherein at least one of the front and rear wheels are provided with an auxiliary steering arrangement and wherein the distribution of torque between the front and rear wheels is selectively controllable.
2. Description of the Prior Art
JP-A-62-8869 discloses a previously proposed 4 wheel drive arrangement wherein the forward wheels of the vehicle are variably supplied with driving torque and wherein the rear wheels are provided with an auxiliary steering system.
The auxiliary steering system is arranged so that, depending on the front wheel steering angle, the steering of the rear wheels is varied with respect to a predetermined steering characteristic.
This arrangement further includes correction means which varies or modifies the steering ratio characteristics. This correction means is responsive to the change in torque distribution and is arranged so that irrespective of the changes in torque distribution, changes in the steering characteristics are prevented by correcting the auxiliary steering angle in a manner wherein the vehicle slip angle is always held essentially at zero.
However, because the rear wheel steering angle is corrected simply on the basis of the torque distribution, it occurs that, in the event of sudden steering under the driving conditions wherein the auxiliary steering angle is set in a manner to promote the ease with which yaw can be induced and the torque distribution device is operated to distribute the driving torque in a manner which tends to resist the augmentation or ease with yaw can be induced, the driving torque tends to be excessively distributed to the front wheels with the result that the steering characteristics undergo an undesirable change.
On the other hand, in the event that driving conditions are such that the auxiliary steering angle becomes set in a manner to inhibit the ease with which yaw can be induced and the torque distribution device is operated to distribute the driving torque in a manner to augment or promote the ease with which yaw is induced, it occurs that the driving torque is largely distributed to the rear wheels with the result that the steering characteristics again undergo an undesirable change.